Bug 20020: use Modern::Perl in XT scripts

Bug 20020: use Modern::Perl in XT scripts

Test Case:
Check the following files have been updated from
use strict;
use warnings;
to
use Modern::Perl;

author/podcorrectness.t
author/show-template-structure.pl
author/translatable-templates.t
author/valid-templates.t
find-license-problems
fix-old-fsf-address
single_quotes.t
verify-debian-docbook.t

Signed-off-by: Jon Knight <J.P.Knight@lboro.ac.uk>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>